Another interesting line of research involves extending quantum causal models to allow for cyclic causal structures[1]}, [2]}. In light of the arguments we have made here, this seems like a promising approach, since one way to model symmetric relationships which can't be decomposed into a strict partial order would be to write them as cyclic structures in the context of a broader causal description. However, we would still caution that the expectation that the underlying structure should be causal (albeit not acyclic) should be treated with care, since these sorts of phenomena do not involve causal order and therefore there is no obvious reason to expect them to be best explicable in the context of a causal formalism, even one that has been generalized in certain ways. For example, ref [2]} continues to maintain that the Bell correlations should be understood in terms of `causation without signalling (i.e., fine-tuned causal influences)' whereas we have argued here that this is not the right way to think of the Bell correlations. This does not mean that research on cyclic causal structures should not be pursued, but it does mean that we should be prepared to be quite flexible about these kinds of models: assumptions that they must have certain prototypically causal features should be carefully examined, and discarded if they are not working well.
